The present invention relates to a non-contact proximity sensor and, more particularly, a proximity sensor having an increased sensing range and capable of sensing magnetic, ferromagnetic, and conductive targets.
A proximity sensor is a device used to detect the presence of an object. The design of a proximity sensor can be based on a number of principles of operation, some examples include: variable reluctance, eddy current loss, saturated core, and Hall effect. Depending on the principle of operation, each type of sensor will have different performance levels for sensing different types of objects. In particular, saturated core and Hall effect sensors are primarily used to detect the presence of magnetic objects, while variable reluctance and eddy current loss sensors are primarily used to detect the presence of metal objects.
Saturated core sensors are known in the prior art. Such proximity sensors typically include a core made from a material that will magnetically saturate when exposed to a magnetic field of a certain flux density. As a magnetic object is moved toward the core assembly, a distance is reached where the magnetic field of the object finds the core to be the smallest reluctance path. As a result, the flux of the field enters the core and, as the distance is decreased the flux density increases and eventually saturates the core. The saturation of the core causes the impedance of the coil to decrease. By measuring changes in the impedance of the coil, the presence of the magnetic object may be detected.
One example of a saturated core sensor can be found in U.S. Pat. No. 4,719,362 to Nest et al. (xe2x80x9cthe Nest Patentxe2x80x9d). The Nest patent discloses an inductive proximity sensor having a core, a conductive coil, and an oscillator circuit. The core is made from a metal that magnetically saturates when exposed to the magnetic field of a target.
Variable reluctance sensors are also known in the prior art. Such proximity sensors typically include a U-type core and coils wound around the core legs. Other typical shapes of variable reluctance proximity sensors cores include: Pot cores, pins, T-cores, E-cores and plates. These devices also typically include an electronic drive device for producing an oscillating electromagnetic field around the coil. Square waves, sine waves, trapezoidal waves, and other unique wave shapes have been used to interface to these sensors. As a permeable object is moved toward the variable reluctance proximity sensor, the permeable object reduces the reluctance of the electromagnetic system, and this variation in the reluctance is measured as a change in the inductance and AC resistance of the coil. When the permeable object moves toward a variable reluctance sensor, the inductance and AC losses of the coil increase.
In some designs, two coils are used, one to produce the electromagnetic field and another to measure the variations in the reluctance. In conventional variable reluctance proximity sensors, it has been common practice to use a ferrite core. The sensing coils are placed on the core to optimize the magnetic field extending to the target material. The core is shaped to contain and extend the electromagnetic field surrounding the coil in a sensing direction or to concentrate or channel the field in other directions, such as behind and to the sides of the coil. One example of a variable reluctance sensor is shown in U.S. Pat. No. 4,387,339 to Akerblom (xe2x80x9cthe Akerblom patentxe2x80x9d). The Akerblom patent discloses an apparatus for measuring the distance between two moving objects.
Eddy current sensors are also known in the prior art. Such proximity sensors have a similar design to variable reluctance sensors in that they typically include Pot cores or U-type core, a coil wound around the center post of the core or the core legs, and an oscillator for producing an oscillating electromagnetic field around the coil. As with the variable reluctance proximity sensor, the eddy current sensor detects the presence of a conducting object by measuring the real and imaginary AC losses of the coil; however, in this design, if the target material is only conductive, the inductance of the coil decreases as the object is moved toward the target.
The present invention provides a proximity sensor that operates as a saturable core proximity sensor, a variable reluctance proximity sensor, and an eddy current proximity sensor. In the preferred embodiment, the core is made of a highly permeable metal. The preferred embodiment provides a durable, low cost, lightweight proximity sensor with increased sensitivity and additional manufacturing benefits.
The precision shaped metal core and the supporting assembly are designed to optimize the sensitivity of the device in three modes of operation. In one mode of operation, the proximity sensor is used to detect the presence of a magnet. When the magnet approaches the sensor, the unique shape and dimensions of the core allow for easy saturation of the core in the magnetic field. When the magnet is close to the sensing device, the core saturates and significantly changes the impedance of a coil configured around the core. A first sensing circuit connected to the coil is used to detect variations in the impedance and configured to provide a signal to indicate the presence of a magnetic object when the coil impedance fluctuates.
In a second mode of operation, the proximity sensor is used to detect the presence of ferromagnetic metals objects, also referred to as permeable metals. In this mode, the coils around the core provide a source field and also provide a means for measuring variations in source field. When a permeable object moves into the source field, the inductance of the coil increases. A second sensing circuit connected to the coil is used to detect variations in the inductance of the coil and configured to provide a signal to indicate the presence of a ferromagnetic target when the coil inductance fluctuates. The highly permeable steel sensor core provides a design with increased sensitivity for detecting ferromagnetic objects at increased ranges.
In a third mode of operation, the proximity sensor is used to detect the presence of conductive objects, such as copper or aluminum. In this mode, the inductance of the coil decreases when a conductive object moves into the source field. The second sensing circuit connected to detect variations in the inductance of the coil. Similar to the variable reluctance mode, when a permeable object moves into the source field, the inductance of the coil increases. A second sensing circuit connected to the coil is used to detect variations in the inductance of the coil and configured to provide a signal to indicate the presence of a ferromagnetic target when the coil inductance fluctuates. The highly permeable steel sensor core also provides a design with increased sensitivity for detecting conductive objects at increased ranges.
A specific core design that can be used in the present invention is a thin, highly permeable metal that is preferably formed from a single piece of sheet metal. The core comprises a substantially flat, rectangular member bent in four locations to form a shape that resembles a rectangular Greek omega character having a head, two legs, and two feet. The bends are right angles with 60 degree bend radii so that the head is perpendicular to the two legs, and the two legs are perpendicular to the two feet. The two feet are parallel to each other, occupy a common plain and also parallel to the head.
The core is positioned in a housing with two inductive coils, each coil being positioned around one of the feet of the core. Another embodiment of the present invention also provides a core and coil assembly with a calibration bolt that is placed through the center of the core to adjust the inductance range of the proximity sensor.
The thin metal structure of the core also provides many other benefits in the manufacturing process of the proximity sensor. Specifically, the thin metal core allows for a more efficient manufacturing process that does not require a considerable amount of machine work, as compared to the prior art U-type metal cores. The core design also provides a rugged, lightweight structure that is not susceptible to great temperature fluctuations. Another advantage of the present invention is that it can be used as a permeable metal target sensor, an eddy-current-loss sensor, or a magnetic field sensor without the need of special preparations to switch the detectors for different types of applications. The proximity sensors of the present invention also provide a core design with increased sensitivity while having a design that is produced in a sufficiently small size.